Downloaded from http://trngl.lyellcollection.org/ at Yale University on July 8, 2015 216 TRANSACTIONS—GEOLOGICAL SOCIETY OF GLASGOW. No. XXII.—A NEW VIEW OF THE ABKAN GBANITE MOUNTAINS. With a Map [Plate V.] and Twenty-two Illustrations, drawn by the author. By JOHN SMITH. [Bead 14th March, 1895.] "This little world, This precious stone set in the silver sea." "Scenes of a truly Alpine character are very rare in Scotland, and perhaps nowhere occur except in the Cuchullin Mountains of Skye and the Granite Peaks of Arran."—PHILLIPS. ALTHOUGH the officers of the Geological Survey have not yet tackled the Arran granite, we have had quite a number of private observers who, from time to time, have written upon the geology of the island, and who, in many cases, merely incidentally as it were, have taken notice of the great granitic area, but, so far as I am aware, no one has yet given the granite itself the special attention which it certainly deserves. A walk through all the Arran granite glens, or even a climb over the tops of the granite hills, is not sufficient to unravel the problem of the structure of the rock, as I know from my experience for some time past. It is only by going round each hill at a height where the granite is bare and free from turf and debris, as well as along the stream courses and over the hills in addition, that the true nature of the rock can be made out, and this task is the one which I, for the past two years, set myself to perform, and the results of which I now propose to give. Dr. Jameson, writing of the Glen Rosa granite in 1798,* says, " The edges of the strata are distinctly perceived, running for a great extent, emerging here and there from below the loose blocks of granite which have fallen from the top of the mountain, or have been formed from the splitting from the surface of the strata." Although the writer uses here the word strata twice in one sentence, he afterwards explains that he thinks the granite is not stratified in the usual meaning of the term. Headrick in 1807t says of Goatfell, "the whole mountain is stratified." Ramsay remarks in 1841,J "The sides also of part of the south * "Mineralogical Tour in the Scottish Isles." By Rev. Dr. Jameson, 1798. t" View of the Mineralogy ... of the Island of Arran." By the Rev. James Headrick, 1807, p. 43. J "The Geology of the Island of Arran, from Original Survey." By Andrew Crranbie Ramsay, 1841, p. 7. Downloaded from http://trngl.lyellcollection.org/ at Yale University on July 8, Trans. Geol. Soc. Glasgo2015w X., Plate THE GRANITIC AREA OF ARRAN. The Dotted Line shows the probable extension of the Granite. The Arrows indicate the dip of the Granite Slabs. The heights of the Mountains are given in feet. Downloaded from http://trngl.lyellcollection.org/ at Yale University on July 8, 2015 SMITH—NEW VIEW OP THE ARRAN GRANITE MOUNTAINS. 217 shoulder of Goatfell, along with many other districts, frequently exhibit a somewhat similar phenomenon, the granite being split into masses of varying sizes, superimposed on each other, and dipping in the direction and at the angle of the descent of the hill, like layers of lava which have flowed over each other, the one being cooled before the eruption of the superincumbent bed. But in the case of the granite, this could not be the case, as it will be afterwards shown that it was formed and consolidated previous to its protrusion through the overlying stratified formations, and consequently no such overflowings could have occurred. It is difficult to assign a reason for this phenomenon, but it seems possible that the melted mass, coming in contact with a cooling substance, viz., the slate, at its surface, refrigerated by degrees downward to the centre, and, as it thus consolidated and con­ tracted, the different layers may have split off from the more heated mass below, in the form which they now assume." Bryce* often mentions the immense sheets and rhombic masses of granite. Speaking of Cir Mohr granite, he says, "It is disposed in irregular tabular masses, split up into rhombic or cuboidal forms by fissures, independent of disintegration, and coeval with the solidification of the rock. The thinner masses we have called sheets; in both, the divisional planes separating mass from mass, and the fissures perpendicular to them, are alike the result of crystallization on the large scale, and bear no analogy to stratifica­ tion, which is the result of sedimentary deposit." Again, he says, "The schistose form has often been described as a true stratification; but this structure is not continuous in one direction as strata are, nor does it exhibit the fracture or incurv­ ation of beds; it is, in truth, but a local modification of the rhombic or cuboid form, under which granite more frequently appears." Landsborough, the younger, writes,f "Should it be resolved to proceed to Cir Mhor, beware of venturing to pass along the Ceims; for a precipice is in the way, rendering this impossible, and these great slabs of granite on its surface are most dangerous, all the more so that they are not very steep, and one is apt to venture * "The Geology of Arran, and the other Clyde Islands." By James Bryce, M.A., LL.D., &c. 3rd ed., 1865, p. 59. t "Arran, and How to See it." By the Rev. David Landsborough. 3rd ed.,.1886,.page 40. Downloaded from http://trngl.lyellcollection.org/ at Yale University on July 8, 2015 218 TRANSACTIONS—GEOLOGICAL SOCIETY OP GLASGOW. upon them. They are slippery, and if once a footing is lost it may be impossible to regain it." I need add no more quotations, my object in giving these being to show that the stratified-looking, tabular, sheet, schistose, or slab-like arrangement of the Arran granite was well known to those who have written about the island. It is, however, strange, that as these "slabs" lie at all angles from a horizontal to a perfectly vertical position, no one has followed out their arrange­ ment to its legitimate issue, as they would have done had the granite been an evidently sedimentary rock. I believe the reason for this is the apparent similarity, although I hope to show that there really is considerable dissimilarity, in the appearances of these granite slab-like forms. Pig. 1. Section—Kilbrannan Sound to Beinn Bharrain. (a) Slate. (b) Fine-grained Granite. (c) Much-jointed Granite. (d) Granite Slabs. In the present paper I propose to follow out all the directions and angles of dip of the slabs, noting all their varieties in every hill and glen within the granitic area, and having done so, to draw the conclusions, which, I think, we are led to by a study of the conditions. It was one day in the summer of 1891, in going up along the AUt Gobhlach (the Y river), that I resolved to trace out the bedded arrangement of the granite. In the course of this stream, which flows directly westwards to Kilbrannan Sound, the slabs are extremely well seen, and resemble, when viewed from a short distance, the courses in a well-bedded sandstone quarry. They dip with the stream, but at a much steeper angle (fig. 1). Downloaded from http://trngl.lyellcollection.org/ at Yale University on July 8, 2015 SMITH—NEW VIEW OF THE ARRAN GRANITE MOUNTAINS. 219 The slate near the line of junction is all but perpendicular, and the granite close up to it is very fine-grained, but coarser at 1^ foot, the particles being very irregular in size. For some distance back from the slate, the granite. has completely lost its slab-like nature, and appears in the deep " gullet" of the burn as a much- joiuted rock, the joints running in all directions. This structure of the granite, as it approaches the slate, will be repeatedly noted in other localities of the area, and its significance interpreted in the summary. From the junction above noted to the base of Beinn Bharrain (the mount of the hilly district) the moor is comparatively flat, and having regard to the manner in which the slabs outcrop in the Allt Gobhlach, one cannot help remarking the immense amount of marine denudation which has taken place along the western face of the granitic area at a time when the sea worked between the 600 feet and the 1300 feet contour lines. Near the western base of Beinn Bharrain the coarse-grained granite slabs dip at from 20° to 30°, and outcrop downwards on the side of the hill. Some of the slabs, here and in the stream, are quite rotten, but for the most part the granite is very solid, the felspar crystals being about \ inch, the quartz ones about \ inch, and those of the biotite \ inch in size, and few in number. Along the shore, near the mouth of the Allt Gobhlach, are many granite boulders, both coarse- and fine-grained. The latter the natives find more difficult to bore than the coarse-grained variety, but, on the other hand, they are much more easily "dressed," and on this account are preferred for building purposes. There are few slate boulders, and those of small size, but the rock is here very shivery and does not lend itself to the formation of boulders. In several exposures between the junction and the shore, the slate is seen to be crumpled, though not to any great extent.